The present invention is generally directed to horizontal penetrations extending between the inner and outer walls of a cryostat, particularly one employing liquid helium as a coolant material. More particularly, the present invention is directed to an insert for this penetration and a horizontal penetration assembly employing such an insert.
In the generation of medical diagnostic images in nuclear magnetic resonance imaging, it is necessary to provide a temporally stable and spatially homogeneous magnetic field. The use of superconductive electrical materials maintained at a temperature below their critical transition temperatures provides an advantageous means to produce such a field. Accordingly, for such NMR imaging devices, a cryostat is employed. The cryostat contains an innermost chamber in which liquid helium, for example, is employed to cool the superconductive materials. The cryostat itself typically comprises a toroidal structure with other nested toroidal structures inside the exterior vessel to provide vacuum conditions and thermal shielding.
Since it is necessary to provide electrical energy to the main coil magnet, to various correction coils and to various gradient coils employed in NMR imaging, it is necessary that there be at least one penetration through the vessel walls. Typical prior art penetrations have been vertical. However, from a manufacturing viewpoint, the construction of vertical penetrations has produced undesirable, problems of alignment and assembly. However, horizontal cryostat penetrations have not been employed for reasons of thermal efficiency. In particular, it is seen that for a coolant such as liquid helium, that there is a large dependency of vapor density upon temperature. Accordingly, helium vapor found within a vertical penetration is naturally disposed in a layered configuration as a result of the density variation from the bottom to the top of the penetration. This layering provides a natural form of thermal insulation along the length of a vertical penetration. In particular, at any position along the axis of such penetration the temperature profile is substantially constant. However, this would not be the case for a horizontal cryostat penetration since any layering that would result would not be in the direction of the long axis of the cryostat penetration. Accordingly, the temperature gradient along the penetration would tend to set up convection currents in the vapor within the penetration. This would result in a much more rapid loss of coolant than is desired. Since the cost of helium is relatively high, it is seen that this loss of coolant is particularly undesirable.
Moreover, as a result of an as yet not fully understood phenomena, it is possible for superconductive windings within the cryostat to undergo a sudden transition from the superconducting state to the normal resistive state. In this circumstance, the electrical energy contained within the coil is rapidly dissipated as resistive (I.sub.2 R) heating of the windings. This can result in a rapid increase in internal helium vapor pressure and accordingly, any cryostat penetration must be provided with pressure relief means.
Furthermore, vacuum conditions are maintained between the innermost and outermost cryostat vessels. If for some reason, a loss of vacuum occurs in this volume, it is also possible to develop a rapid increase in the coolant vapor pressure. For this reason also, pressure relief means are desirable for cryostat penetrations.
Accordingly, it is seen that because of the large density changes between cold and warm helium, free convection secondary flows are easily set up in horizontal cryostat penetrations. These flows considerably degrade the thermal efficiency of horizontal penetration. If the penetration is densely packed with foam or equipped with a vapor cooled, thermally efficient blowout plug, pressure relief of the vessel could be obstructed by frost buildup in the vapor cooled channel. It is therefore seen that horizontal cryostat penetrations for NMR magnet cryostats require thermally efficient inserts that supress free convection flow. These inserts must also provide sufficient exhaust area to relieve internal vessel pressure in case of magnet quench or vacuum loss.